Time base fluctuation of waveforms, that is, jitter, normally occurs in video signals reproduced by recording-reproduction apparatus. It is necessary to correct the jitter in order to ensure stability of the reproduced image. Now, some magnetic recording-reproduction apparatuses such as VTRs used in broadcasting, employ a direct FM recording method. According to this method, video signals are recorded and reproduced by directly modulating carrier frequency based on video signals of the NTSC color system and other system. Here, in order to correct jitter, a color burst signal inserted at each horizontal scanning cycle of the video signal serves as a reference signal and a jitter detection signal is formed based on the time base fluctuation of specified zero-cross points of the color burst signal.
Some magnetic recording-reproduction apparatuses, such as home VTRs, use the down converted chrominance. component signal recording method. In this case, a color burst signal is not added to the video signal. Accordingly, normally, a jitter detection signal is produced by detecting specified portions, such as a rising edge or falling edge, of a horizontal sync signal, and jitter is corrected according to this jitter detection signal.
However, random noise gets superimposed on the horizontal sync signal and this noise lowers the accuracy of the jitter detection signal. Since this lowering in accuracy of the jitter detection signal makes sufficient correction of jitter difficult, destabilization of the reproduced image is the result.
A method for accurately detecting jitter that gets included in the reproduced signal is disclosed, for example, in the 1988 Japanese Laid-Open Publication 274290 (Tokukaihei 63-274290). This method makes use of a phenomenon that the jitter that occurs during reproduction of the video signal also causes jitter that occurs during reproduction of a carrier (referred to hereinafter as FM carrier) which is frequency-modulated according to the video signal.
In this case, the phase of the FM carrier is reset for each horizontal scanning cycle in response to the leading edge of the horizontal sync pulse included in the video signal. (This method is referred to hereinafter as the carrier reset method.) That is, during recording of the video signal, the FM carrier is recorded not with the phase completely independent of that of the video signal, but with the phase having been reset to the reference phase in response to the leading edge of the horizontal sync pulse. Further, during reproduction of the video signal, a horizontal sync pulse is demodulated from the reproduced FM carrier, and the reproduced FM carrier that has been gated by the horizontal sync pulse is used as a reference burst signal. With this arrangement, jitter can be corrected by the use of a jitter detecting signal with high accuracy that is identical to the jitter detecting signal formed from a reference burst signal in a magnetic recording-reproduction apparatus of the direct FM recording method.
However, in the above-mentioned conventional magnetic recording-reproduction apparatuses wherein the carrier resetting method is employed, since the carrier resetting operation in response to the leading edge of the horizontal sync pulse is conducted during frequency modulation, a discontinuous phase in the FM carrier occurs due to a phase shift caused by the carrier resetting operation. Here, explanation is given of the phase shift.
As shown in FIG. 14, when the above carrier resetting operation is performed with a predetermined time delay .pi. from the leading edge of a horizontal sync pulse, the timing of the leading edge of the horizontal sync pulse is given by t.sub.0 '; that of the carrier resetting, t.sub.0 ; that of the next leading edge, t.sub.1 '; and that of the next carrier resetting, t.sub.1. Further, the FM carrier amplitude is represented by A, and the instantaneous value of the FM carrier amplitude is represented by F(t). As shown in FIG. 15, the video signal peak value that is measured on the basis of the peak level (normally set to a negative value) of the horizontal sync pulse is represented by V.sub.0, while the instantaneous video signal amplitude value is represented by V(t). In this case, the relationship between F(t) and V(t) is shown as a function of time t to form the following equation (1). ##EQU1##
Here, the following equation holds: EQU .phi.(t)/(2.pi.)=f.sub.TP +.phi..sub.0 /(2.pi.)+(.DELTA.f/V.sub.0).intg.V(t)dt
where; f.sub.TP is the FM carrier frequency that is frequency-modulated according to the peak level of the horizontal sync pulse; and .DELTA.f represents the frequency deviation that shows a difference between the FM carrier frequency that is frequency-modulated according to the peak value V.sub.0 of the video signal and the above frequency f.sub.TP.
Accordingly, the phase shift .phi..sub.1 that is caused by the carrier resetting is indicated by the following equation (2). ##EQU2##
Moreover, in the case of phase shifting, since the frequency f.sub.TP is given by an integer multiple of the horizontal scanning frequency f.sub.H ; therefore, f.sub.TP (t.sub.1 -t.sub.0) becomes an integer number and can be omitted. Thus, the above-mentioned phase shift .phi..sub.1 is expressed by the following equation (3). ##EQU3##
When the carrier resetting operations are conducted during recording the FM carrier of FIG. 14 in a magnetic tape, suppose that magnetization patterns (i),(ii),(iii), (iv),(v),(vi),(vii) or (viii) are produced as shown in FIG. 16. In FIG. 16, phase shift amounts of the FM carrier at the time of the carrier resetting are illustrated as if they varied in the order of drawings in FIG. 16; however, the video signal integral value in the above-mentioned equation (3) virtually varies at random for every horizontal scanning cycle, the phase shift amount also varies at random.
Accordingly, when the above-mentioned magnetization patterns (i) through (viii) are read out as the FM carrier and frequency demodulated, overshoots (ii).multidot.(iii).multidot.(iv) and undershoots (v).multidot.(vi).multidot.(vii).multidot.(viii) occur at random as shown in FIG. 17 due to the discontinuous phases of the FM carrier at the time of the carrier resetting. (Here, Roman figures used in FIG. 17 correspond to those used in FIG. 16.) In the case of adopting the sync peak clamping system for clamping the video signal at peak levels of the horizontal sync pulses in order to keep the black level of the reproduced video signal, these overshoots and undershoots cause clamping errors.
On the other hand, when the carrier resetting is operated coinciding with the leading edge of the horizontal sync pulse, the above-mentioned overshoots (ii).multidot.(iii).multidot.(iv) and undershoots (v).multidot.(vi).multidot.(vii).multidot.(viii) cause transient distortions, as shown in FIG. 18, near the leading edge of the horizontal sync pulse that has been frequency-demodulated. These transient distortions cause variations in rising time of the leading edge of the horizontal sync pulse, and thereby the phase accuracy at the leading edge of the horizontal sync pulse is adversely affected.
In this case, since the phases of the FM carrier are lined up at the trailing edge of the horizontal sync pulse, no problem is encountered in the normal operation. However, since the processing operation for correcting jitter is limited to and applied only at the trailing edge of the horizontal sync pulse, the correcting operation is delayed by a pulse width from the leading edge of the horizontal sync pulse to the trailing edge thereof. Further, the transient distortions cause variations in the horizontal sync pulse width, and thereby the correlation between the leading edge of the horizontal sync pulse and the trailing edge thereof is adversely affected.
As described above, in the conventional magnetic recording-reproduction apparatus which conducts the carrier resetting operation, jitter can be corrected with high accuracy by obtaining the reference burst signal from the reproduced FM carrier. However, since a discontinuous phase shift occurs in the FM carrier upon the carrier resetting operation, a problem is encountered in that transient distortions such as overshoots and undershoots are caused in the horizontal sync pulse that has been frequency-demodulated.
In the 1989 Japanese Laid-Open Publication 264492 (Tokukaisho 1-264492), a magnetic recording-reproduction apparatus is disclosed. This device has a recording system which, as shown in FIG. 19, inserts a phase compensation pulse into the front porch of the video signal in order to prevent transient distortion of a horizontal sync pulse section due to carrier reset. In other words, as shown in FIG. 20, the recording system of this magnetic recording-reproduction apparatus:
(a) detects the phase of the FM carrier corresponding to the front porch of the video signal from the frequency modulator;
(b) produces a phase compensation pulse which corresponds to the detected phase, the height of the pulse being preliminarily estimated; and
(c) enters the phase compensation pulse into the frequency modulator and inserts it into the front porch of the video signal.
The frequency of the FM carrier (referred to hereinafter as FM frequency) is corrected according to this phase compensation pulse. As a result, discontinuity of the phase of the FM carrier is reduced.
However, in this magnetic recording-reproduction apparatus, high-speed feedback becomes necessary since the insertion of the phase compensation pulse is carried out immediately after the detection of the phase of the FM carrier corresponding to the front porch of the video signal. As a result, a sophisticated and expensive circuit configuration becomes necessary. Further, in this magnetic recording-reproduction apparatus, since the phase of the FM carrier varies freely at the front porch for each horizontal scanning period, the pulse height of the the phase compensation pulse before horizontal scanning cannot be estimated by the use of the phase of the FM carrier. Consequently, it becomes difficult to achieve a phase compensation pulse having a pulse height which sufficiently eliminates phase distortion.
A method is known according to which a high-frequency carrier is frequency-modulated based on the video signal, thereby producing a primary FM carrier. Then, a center frequency of the primary FM carrier is shifted to a low frequency, and a low-frequency FM carrier (secondary FM carrier) produced according to this shift is recorded. Conventionally, the frequency of the frequency-modulated carrier is up to 15 MHz. This method has the advantage over the method wherein the low-frequency carrier is directly frequency-modulated and recorded, in that unwanted higher harmonic wave components do not get included easily, and in that linearity is also good. This method is particularly effective when frequency modulation of high-frequency carriers is carried out based on wide-band video signals such as High-Definition Television signals.
However, a disadvantage exists in that, during frequency modulation even if the carrier reset is applied to the primary FM carrier, the phase shifts simultaneously with the shifting of the frequency to the low-frequency band. As a result, the phase of the secondary FM carrier no longer remains synchronous with the horizontal sync pulse.
On the other hand, in the method wherein the carrier of the low-frequency is frequency-modulated directly, unwanted higher harmonic wave components get included in the FM carrier and the problems to be described later arise.
For example, a frequency demodulator in a video tape recorder (referred to hereinafter as VTR) as shown in FIG. 21 has a delay line driver 201, a full wave rectification circuit 202 and a delay line 204 connected respectively to outputs of the delay line driver 201, and a low-pass filter (LPF) 203 connected to the output of the full wave rectification circuit 202. The termination of the delay line 204 is short-circuited.
Now, consider an ideal case, as shown in FIG. 22(b), of an FM carrier being recorded and reproduced in which no unwanted higher harmonic wave components get included, the FM carrier being based on a video signal having a waveform as shown in FIG. 22(a). The reproduced FM carrier is formed into a square wave on passing through a high gain limiter circuit or the like and then is supplied to the delay line driver 201. The delay line 204 delays and reflects the square wave. Then, a composite wave, consisting of the square wave and the delayed and reflected square wave, is supplied to the full wave rectification circuit 202. Accordingly, a pulse signal whose leading edge is synchronous with the zero-cross points of the FM carrier is supplied from the full wave rectification circuit 202 to the LPF 203, as shown in FIG. 22(c). The carrier wave component of the pulse signal is eliminated by the LPF 203, and an ideal video signal with no distortion is demodulated, as shown in FIG. 22(d).
As against this, in the case where a low-frequency carrier is directly frequency-modulated, as shown in FIG. 22(e), the reproduced FM carrier becomes a square wave because unwanted higher harmonic wave components get included. Moreover, since the frequency of the reproduced FM carrier shifts due to superimposition of the higher harmonic wave components, the rising edge of the square wave shown in FIG. 22(e) is no longer synchronous with the zero-cross points of the ideal FM carrier shown in FIG. 22(b). As shown in FIG. 22(f), a pulse signal is supplied to the LPF 203 from the full wave rectification circuit 202. The pulse signal is synchronous with the zero-cross points of the deviated FM carrier which includes the higher harmonic wave components. As a result, distortion occurs in the video signal demodulated after the elimination of the carrier wave component of the pulse signal by the LPF 203, as shown in FIG. 22(g). Consequently, a problem occurs in that moire increases.
Moreover, among the higher harmonic wave components which get included in the reproduced FM carrier, the tertiary higher harmonic wave component is the main cause of distortion in the demodulated signal (the tertiary higher harmonic wave component is a spectrum component of the reproduced FM carrier and its center frequency and modulation index are three times the center frequency and modulation index of the reproduced FM carrier). An explanation follows regarding this tertiary higher harmonic wave component, using specific values.
Suppose that a carrier having a 13 MHz grey level frequency, shown in FIG. 23(b), has been frequency-modulated based on a MUSE signal, shown in FIG. 23(a), having a 9 MHz frequency band. The deviation after frequency modulation is assumed to be .+-.3.5 MHz and the degree of pre-emphasis on the MUSE signal is regarded to be 12 dB (approximately 4 times). If the frequency band of the FM carrier is 2.times.(deviation + frequency band of the video signal), in the present case, the upper-limit value (shown in FIG. 23(b)) of the frequency band of the FM carrier is EQU 13+(3.5.times.4+9)=36 [MHz] (1)
Further, the center frequency (shown in FIG. 23(c)) of the tertiary higher harmonic wave component is EQU 13.times.3=39 [MHz]
The upper-limit value (shown in FIG. 23(c)) and the low-limit value (not shown) respectively are EQU 13.times.3+(3.5.times.4.times.3+9)=90 [MHz] EQU 13.times.3-(3.5.times.4.times.3+9)=-12 [MHz] (2)
(broken lines in FIGS. 23(b) and (c) show the respective negative folding components).
As will be clear on comparing (1) and (2) above and referring to FIGS. 23(b) and (c), in the case where the frequency of the carrier is low, the tertiary harmonic wave component enters the transmission band of the FM carrier, and as a result cannot be isolated even by the LPF 203. FIG. 23(d) shows the frequency band of the FM carrier which should be recorded after the high-frequency band cut by the LPF 203; the broken line shows the unwanted tertiary higher harmonic wave component in the transmission band of the FM carrier.
Consequently, since the unwanted tertiary higher harmonic wave component and the negative folded higher harmonic wave components adversely affect frequency demodulation, the distortion of the demodulated video signal increases, as does moire. Furthermore, in the case where the distortion of the video signal is large, demodulation can become impossible. Similar problems occur in pulse-counter type frequency modulators used in conventional VTRs as well.